The preparation of siloxane-oxyalkylene polyether copolymers ("Copolymers") by the hydrosilylation reaction of an organohydridosiloxane and an unsaturated polyoxyalkylene (polyether) is well known in the art. The Copolymers may be prepared from allyl, methallyl or propargyl started polyethers and polydimethylhydridosiloxanes in the presence of solvents as described in U.S. Pat. Nos. 3,980,688 and 4,025,456, or under solventless conditions with specific additives as disclosed in U.S. Pat. Nos. 4,847,398, 5,191,103 and 5,159,096. Whatever the method or process, it is essential that the polyethers be free of contaminants which can inhibit the rate, selectivity and completeness of the catalytic hydrosilylation.
Although it is known that oxidation impurities in the polyethers inhibit the hydrosilylation of the polyethers, the exact identities of these inhibitors are unknown. They are believed to include acetal hydroperoxides, allyl hydroperoxides and free radicals localized at the tertiary carbon atoms in the hydrophobic segments (e.g., propylene oxide) of unsaturated polyethers. Oxidation impurities are most likely to occur in polyethers which have been stored for a long period with no or insufficient quantities of antioxidant. However, they may also be present in freshly prepared polyethers which may have gotten too hot in the presence of air or oxygen.
The hydrosilylation activity of polyethers containing oxidation impurities appears to depend on the specific hydroperoxides present in the polyether. In some cases, less than 100 parts per million (ppm) hydroperoxides will inhibit hydrosilylation, while in others as much as 2500 parts per million (ppm) hydroperoxides is tolerable. Indeed, the use of organic hydroperoxides as temporary catalyst inhibitors in the addition-cure synthesis of silicone gels and elastomers is disclosed in U.S. Pat. No. 4,061,609.
For example, propanal, even at low (ppm) levels is a source of unacceptable quality in the copolymer product. When the polyethers are uncapped, the Copolymer can have a higher than expected viscosity, or may even gel, on account of crosslinking (acetal formation) between the hydroxyl groups and the propanal. As disclosed in U.S. Pat. No. 4,847,398, sodium carboxylates can control allyl-propenyl isomerization and act to prevent the propenyl ether cleavage resulting in propanal formation.
Sodium borohydride, NaBH.sub.4, is known to be effective for the decomposition of peroxides in polyethers. The polyethers are made suitable for hydrosilylation by this treatment, but experiments have shown that the resulting Copolymers are not necessarily acceptable.
Unrelatedly, an extensive prior art already exists on the use of ascorbic acid, its alkali metal salts and ascorbic acid esters as antioxidants in foods and medicines. Citric acid, its salts and esters have also been used as food additives, primarily for pH and flavor control. However, there appears to be no teachings regarding the use of these acids or their derivatives with industrial chemicals.